The present invention relates generally to a hybrid propulsion system for aircraft, and more particularly to a gas turbine-based propulsion system that can provide primary motive power to the aircraft over a portion of the aircraft's flight envelope, and a stored electric-based power system that is selectively coupled to the gas turbine such that during other portions of the aircraft's flight envelope, the electric-based power system can provide the aircraft with its primary motive power.
Aircraft are broadly categorized as either fixed-wing vehicles (such as an airplane) or rotary-wing vehicles (such helicopters and autogyros, the latter also referred to as autogiros or gyrocopters). Gas turbine engines are widely used to power both the fixed-wing and rotary wing forms of aircraft, where fixed-wing vehicles often employ turbofan, turbojet and turboprop variants, and rotary-wing vehicles often employ turboshaft variants. In all circumstances, the basic gas generator hardware is common, including a compressor, a combustor and turbine, where the compressor and turbine rotate on a generally common shaft (or set of concentric shafts) such that energy extracted from the turbine is used to power the compressor. Turbofans are very similar to turbojets, with the exception that they typically include an additional fan located upstream of the compressor. A turboprop engine, in addition to including the respective turbofan or turbojet componentry, also includes a fore-mounted drive shaft that spins in common with the shaft of the compressor and turbine. To match the high rotational speed of the compressor and turbine to that of a propeller, a gearbox is inserted between the front end of the drive shaft and a propeller shaft. Turboshafts also include similar components to the turbofans and turbojets, and additionally include a shaft rotatably responsive to another turbine stage.
In turboshaft engines, power generated by the gas generator (which is spinning about a generally horizontal axis) is transferred to the shaft (which is spinning about a generally vertical axis such that it can turn a rotor made up of a series of blades that radially extend from a central hub) through a gearing mechanism, such as a bevelled or worm gear. Shaft horsepower needs to produce a particular rotor rotational speed varies depending on the aircraft type, size and intended mission. For example, the CH-47 Chinook is a popular twin-rotor helicopter designed for commercial and military heavy lifting. Rotor blade power requirements for helicopters such as this may be in the range of five thousand horsepower, while speed requirements of around two hundred and twenty five revolutions per minute (RPM) are typical.
Despite their widespread use, conventional gas turbine-based propulsion systems have significant drawbacks for certain types of aircraft. For example, in the event a turboshaft engine fails, a helicopter, gyrocopter or other inherently unstable aircraft has no way of returning to earth under its own power, and at best can expect to have to endure a controlled crash landing. Likewise, if a helicopter employing a turboshaft engine as propulsive power is flying or hovering over an area where terrorists, armed conflict or related hostilities exist, the extreme heat put out by the engine or engines may make the helicopter exceedingly vulnerable to attack from infrared (IR) seeking weaponry. Accordingly, there exists a need for a propulsion system that overcomes these shortcomings.